Down syndrome, also known as trisomy 21 or Down syndrome, is a chromosomal abnormality that manifests itself as an abnormality of chromosome 21, with standard, translocation and chimerism. 60% of affected children are aborted in early fetal life, and those who survive have significant mental retardation, peculiar facial features, growth disorders and multiple malformations. The causes of Down’s syndrome are: (1) Advanced pregnancy: More than 90% of Down’s children are of the standard type, and the majority originate from the non-segregation of chromosome 21 at the time of egg formation in the affected mother. The decline of ovarian function is positively correlated with age. As the ovaries of older pregnant women tend to age, germ cells or fertilized eggs are prone to chromosomal nondisjunction or chromosomal aberrations during cell division. (2) Inadequate folic acid supplementation: Folic acid deficiency in pregnant women is an important cause of neural tube defects in fetuses. folic acid deficiency or metabolic disorders can cause DNA hypomethylation, which in turn can lead to chromosomal nondisjunction DNA. women supplement folic acid in early pregnancy to increase the concentration of folic acid in plasma and maintain chromosomal stability, which can reduce the incidence of Down’s syndrome. (3) Pre-pregnancy fever: It has been suggested that fever caused by pathogenic microbial infections can cause harm to the mother, and the drugs used to treat the infection may also have adverse effects on certain chromosomes during the specific period of embryo formation, leading to the occurrence of fetal birth defects. (4) Multiple abortions: Multiple abortions (3 or more) are a risk factor for the development of Down’s syndrome. (5) Environmental factors: Activities such as home renovation or purchase of a new car, makeup, etc. can expose the mother to chemicals such as benzene, and some studies have confirmed that the rate of chromosomal aberrations is significantly higher in benzene exposed individuals than in non-exposed individuals. Main manifestations: (1) External signs: The child’s face is often different from normal, with a relatively dull face, widened eye spacing, crooked eye slits, and low ear position. The child’s tongue is short and fat, and he or she constantly spits out his or her tongue and drools. Height and head development are less than normal, and the occipital region is flat. The skin is lax, the hair is thin and soft, the bones are severely delayed, the teeth grow later than normal, and the teeth are easily misaligned. The limbs and fingers (toes) are short and stubby, and the joints are abnormally flexible and can be severely bent. The trigeminal point of the palm of the hand is biased toward the distal end of the body, and the palm lines of the hand are often of the through type. The gap between the bunion and the second toe is too large (straw shoe foot), and the lines on the bunion of the affected child are mostly bow-shaped. (2) Mental retardation: The child’s mental retardation becomes more pronounced with age, with an IQ of 25-50. (3) Developmental retardation: Some children have poorly functioning hearts or other organs; low immunity and susceptibility to pathogenic microorganisms; and are tens of times more likely to develop leukemia than normal people. Patients who survive beyond the age of 30 often develop symptoms of Alzheimer’s disease. (4) Late sexual maturity: male patients are sterile for life, while female patients may experience menstruation during puberty and may have the possibility of having offspring. There is no effective treatment for Down’s syndrome, so pregnant women must be screened for Down’s syndrome before delivery, and the risk rate can be calculated through blood sampling. For high-risk pregnancies, further non-invasive DNA or amniocentesis tests are required. If you are an older pregnant woman, or have a Down’s child in your family, or have had a Down’s child, you can avoid the birth of a Down’s child by not having a Down’s screening test and having a non-invasive DNA or amniocentesis test directly.